Scanning electron microscope

ABSTRACT

In a scanning electron microscope, slimming is reduced by reducing a frame count. As the frame count is reduced, the amount of detected secondary electrons decreases, so that a probe current amount is increased to emit an increased amount of detected secondary electrons. A primary electron beam is scanned on a sample, a histogram is created, and the histogram is second-order differentiated to calculate a level of halftone at which a sample image changes in contrast, and to calculate the probe current amount. By adjusting the frame count suitable for the calculated probe current amount, and the contrast suitable for the sample image, the slimming of the sample is limited, and a highly visible sample image is generated for length measurement.

BACKGROUND OF THE INVENTION

The present invention relates to a scanning electron microscope forobserving a miniature pattern to measure dimensions thereof, and moreparticularly, to a scanning electron microscope for observing andmeasuring a sample, the shape of which can be deformed by an electronbeam irradiated thereto.

Scanning electron microscopes (SEM) are widely used in manufacturing andtesting steps of a functional product such as a semiconductor device, athin film magnetic head, and the like, which are fabricated bymicro-machining the surface thereof, for measuring widths of processedpatterns and inspecting the appearance of resulting products. Thescanning electron microscope is an apparatus for forming the image of asample by narrowing down an electron beam emitted from an electronsource with a converging lens or an objective lens which makes use of aninteraction of a magnetic field or an electric field with the electronbeam, one-dimensionally or two-dimensionally scanning the electron beamon the sample using a deflector, detecting a secondary signal (secondaryelectrons, reflected electrons, or electromagnetic waves) with adetector which makes use of an opto-electric effect or the like, andconverting the detected signal into a viewable signal such as aluminance signal synchronized to the scanning of the electron beam.Considerable efforts have been put into the scanning electron microscopeto provide a sample image which accurately corresponds to the shape ofthe surface of the sample under observation and length measurement, andthe distance between arbitrary two points is calculated on the surfaceof the sample from the sample image thus generated. This calculation iscommonly called “length measurement,” and a scanning electron microscopehaving such a calculation function is called a “length measuring SEM.”Such a scanning electron microscope irradiates the surface of a sampleunder observation with an electron beam having accessible energy ofseveral hundreds of electronvolts, as a matter of course.

On the other hand, further miniaturization has been advanced in recentyears in the micro-machining on the surface of semiconductor, and aphotoresist which reacts to argon fluoride (ArF) excimer laser light(hereinafter called the “ArF resist”) has been used for a photosensitivematerial of photolithography. Because of its wavelength as short as 193nm, the ArF laser light is regarded as suitable for exposure to moreminiature circuit patterns. However, the results of recentinvestigations have revealed that the ArF resist is highly vulnerable toelectron beam irradiation, and when a formed pattern is observed ormeasured with a scanning electron microscope, the scanning of aconverged electron beam causes a condensation reaction in a base acrylicresin or the like, resulting in a reduction in volume (hereinaftercalled “slimming”) and an eventual change in the shape of a circuitpattern.

It is said that for reducing the slimming of the ArF resist, it iseffective to reduce an irradiation density of an electron beam to asample. However, a reduction in the irradiation density of an electronbeam causes a reduction in the amount of secondary electrons generatedfrom the sample, resulting in a dark image. Therefore, there is a needfor a method of limiting the slimming and generating a highly visibleimage.

For generating a highly visible image, an optimal brightness andcontrast must be defined for a sample image. In the prior art, thebrightness and contrast of a sample image are adjusted by changingcondition settings for a detector and an amplifier. JP-A-7-240166describes a contrast adjusting method using a contrast level conversionfunction.

In the prior art method mentioned above, when a micro-machined ArFresist pattern is measured twice under the conditions of a frame countequal to 16, and a probe current amount equal to 24 pA, a slimmingamount of 1.3 nm occurs between the first and second lengthmeasurements, demonstrating a failure in sufficiently reducing theslimming in the length measurement of the micro-machined ArF resistpattern. Disadvantageously, the prior art method of reducing theslimming does not take into consideration an image control techniquewhich relies on the relationship between the number of times of electronbeam scanning required for creating a sample image (hereinafter calledthe “frame count”) and the probe current amount, and fails tosufficiently reduce the slimming and generate a highly visible sampleimage.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a scanning electronmicroscope which is capable of reducing the influence of slimming toproduce a highly visible sample image when it is used to observe ormeasure a sample, such as an ArF resist, which is vulnerable to electronbeam irradiation and can suffer from slimming.

An experiment made by the inventors found that the slimming of the ArFresist is deeply related to a frame count, and that the probe currentamount and an accelerating voltage do not significantly affect theslimming. While it has been conventionally believed that the slimming isaffected by the electron beam irradiation density, the result of theexperiment convinces that the ArF resist has different properties fromother resists.

FIGS. 2A–2C show the relationship between the number of times of lengthmeasurements and slimming when an ArF resist having a consistent patternwas measured while a frame count, the probe current amount, and anaccelerating voltage were varied. FIG. 2A shows the relationship betweenthe slimming and the number of times of length measurements when theframe count was varied under the conditions of the accelerating voltageequal to 500 V and the probe current amount (current amount of primaryelectron beam) equal to 24 pA. It can be seen in FIG. 2A that theslimming is reduced as the frame count is smaller. It is understood thatas the frame count is reduced to one half, the slimming can be reducedby 2–2.3 nm each time the frame count is incremented by one. FIG. 2Bshows the relationship between the slimming and the number of times oflength measurements when the probe current amount is varied under thecondition of the frame count equal to eight, and the acceleratingvoltage equal to 500 V. It can be said that even though the probecurrent amount is varied, the slimming is not significantly affected bythe variations in the probe current amount, taking into account thelength measurement repeatability (3σ). FIG. 2C shows the relationshipbetween the slimming and the number of times of length measurements whenthe accelerating voltage was varied under the conditions of the framecount equal to eight, and the probe current amount equal to 24 pA. As isthe case with the probe current amount, it can be said that variationsin the accelerating voltage does not either significantly affect theslimming, taking into account the length measurement repeatability.

From the foregoing results, it is understood that a reduction in theframe count is most effective for reducing the slimming. As mentionedabove, it has been conventionally said that the slimming is effectivelyreduced by reducing the electron beam irradiation density, i.e.,reducing the frame count in this case, and by reducing the probe currentamount. However, with the ArF resist, it is concluded that while theframe count does affect the slimming, the probe current amount does notaffect the slimming. This result is different from the conventionalcommon sense.

However, when the frame count is reduced for reducing the slimming, aninsufficient number of times of scanning for creating a sample imagecauses a smaller amount of detected secondary electrons, which wouldgive rise to such problems as lower contrast and the generation of noisedue to a lower S/N ratio. To avoid these problems, the present inventionincreases the probe current amount, which does not significantly affectthe slimming, to detect an increased amount of secondary electronsemitted from a sample, thereby ensuring an image quality equivalent to asample image generated by the conventional method, even though the framecount is reduced (hereinafter called the “low frame scanning scheme”).

In the prior art, a histogram is created for a generated sample image,the average of the histogram is defined as the brightness of the sampleimage, and the brightness is controlled by increasing the dynamic rangeof an amplifier. Then, the standard deviation of the histogram isdefined as the contrast of the sample image, and the contrast iscontrolled by a detecting condition of a detector. With thisconventional control method, when the low frame scanning scheme isperformed, the histogram largely shifts due to large variations in theprobe current amount, so that the sample image must be scannedrepeatedly to control the histogram, and the histogram must be createdagain. Even if the slimming is reduced by reducing the frame count, therepeated scanning of the electron beam, for creating the histogram,results in an increased amount of slimming. A need therefore exists fora method of reducing surplus electron beam scanning by minimizing thenumber of times the histogram is created and calculating a brightnessand contrast optimal for a sample image.

In the present invention, therefore, the luminance of each pixel in asample image generated by electron beam scanning is divided, forexample, into 256 levels of halftone to create a luminance histogramwhich indicates the number of pixels at each level of halftone. Then,the luminance histogram is relied on to calculate the probe currentamount and the frame count which provide appropriate contrast andbrightness of the image. The calculated probe current amount and framecount are set in the apparatus to form a sample image for measurement.

Specifically, a scanning electron microscope of the present inventionincludes an electron beam source, a lens system for converging a primaryelectron beam emitted from the electron beam source onto the surface ofa sample, a scanning unit for tow-dimensionally scanning the convergedprimary electron beam on the surface of the sample, a detector fordetecting a secondary signal generated from the sample irradiated withthe primary electron beam, an amplifier for amplifying a signal detectedby the detector, a drawing unit for forming a sample image based on thesignal amplified by the amplifier, and a control processing unit. Thecontrol processing unit creates a luminance histogram of a sample image,controls a current value of the primary electron beam such that thebrightness of the image calculated from the luminance histogram islocated substantially at the center of an overall halftone width, andcontrols an amplifying condition for the amplifier to provide a contrastcorresponding to the brightness of the image.

The control processing unit typically derives a function from theluminance histogram, draws a curve representative of the function,calculates two halftone levels at which a second-order differential ofthe function takes maxima in a lower halftone region and a higherhalftone region of the curve, and controls the amplifying condition forthe amplifier such that the two halftone levels are included in apredetermined halftone range.

When the curve representative of the function derived from the histogramhas a peak in a lower halftone end region, the control processing unitcreates a virtual histogram by replacing the peak region of thehistogram with a tangential line which is tangential to a lower halftoneregion of the curve and intersects with a halftone level axis, andcontrols the current value of the primary electron beam such that thebrightness of the image calculated from the virtual histogram is locatedsubstantially at the center of the overall halftone width. In thisevent, the control processing unit controls the amplifying condition forthe amplifier such that a halftone level at which the tangential lineintersects with the halftone level axis, and a halftone level at whichthe second-order differential takes a maximum in a higher halftoneregion of the curve are included in the predetermined halftone range.

When the curve representative of the function derived from the histogramhas a peak in a higher halftone end region, the control processing unitcreates a virtual histogram by replacing the peak region of thehistogram with a tangential line which is tangential to a higherhalftone region of the curve and intersects with the halftone levelaxis, and controls the current value of the primary electron beam suchthat the brightness of the image calculated from the virtual histogramis located substantially at the center of the overall halftone width. Inthis event, the control processing unit controls the amplifyingcondition for the amplifier such that a halftone level at which thetangential line intersects with the halftone level axis, and a halftonelevel at which the second-order differential takes a maximum in a lowerhalftone region of the curve are included in the predetermined halftonerange.

The electron beam microscope of the present invention creates ahistogram of a sample image, and can find a minimum frame count from acalculated probe current amount based on halftone levels near the borderof a sample and a background. By observing the sample under theconditions of the calculated probe current amount and the minimum framecount, a highly visible image can be displayed while limiting theslimming of the sample. In addition, the throughput can be improved bythe low frame scanning scheme with the minimum frame count and thehistogram control method of the present invention.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram generally illustrating an electron microscopewhich has a low frame length measurement function according to thepresent invention;

FIG. 2A is a graph showing the relationship between the number of timesof length measurements and the slimming when a frame count is varied;

FIG. 2B is a graph showing the number of times of length measurementsand the slimming when the probe current amount is varied;

FIG. 2C is a graph showing the relationship between the number of timesof length measurements and the slimming when an accelerating voltage isvaried;

FIG. 3 shows an example of a luminance histogram of a sample image, anda first-order differential graph and a second-order differential graphof the histogram;

FIGS. 4A–4C are graphs generally showing an exemplary histogram controlmethod;

FIGS. 5A–5C are graphs generally showing another exemplary histogramcontrol method; and

FIG. 6 is a flow chart illustrating a procedure for image acquisitionbased on the histogram control length measurement using the image.

FIG. 7 is a diagram illustrating an example of display screen of sampleimage unit.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, embodiments of the present invention will be describedwith reference to the accompanying drawings.

FIG. 1 is a block diagram generally illustrating a scanning electronmicroscope according to the present invention. A voltage is appliedbetween a cathode 101 and a first anode 102 by a high voltage controlpower supply 104 which is controlled by a control processing unit 119 todraw a predetermined emission current from the cathode 101. Since anaccelerating voltage is applied between the cathode 101 and a secondanode 103 by the high voltage control power supply 104 controlled by thecontrol processing unit 119, a primary electron beam 110 emitted fromthe cathode 101 is accelerated to travel to a subsequent lens system.The primary electron beam 110 is converged by a convergence lens 105controlled by a convergence lens control power supply 106, and anunnecessary region of the primary electron beam 110 is removed by anaperture plate 107. Then, the primary electron beam 110 is converged byan objective lens 111 controlled by an objective lens control powersupply 112 into a miniature spot which is two-dimensionally scanned on asample 113 by a deflection coil 108. A scanning signal of the deflectioncoil 108 is controlled by a deflection coil control power supply 109 inaccordance with an observation scaling factor. The sample 113 is fixedon a sample stage 114 in such a manner that it is two-dimensionallymovable. The sample stage 114 is moved by a stage controller 115.Secondary electrons 116 emitted from the sample 114 irradiated with theprimary electron beam 110 are detected by a secondary electron detector117, and amplified by an amplifier 118. A rendering unit 120 convertsthe detected secondary electron signal to a visible signal which isappropriately arranged on a different plane, thereby displaying an imageadapted to the surface shape of the sample 113 on a sample display unit121. An input unit 122 interfaces the operator with the controlprocessing unit 119, such that the operator gives instructions throughthe input unit 122 to control the aforementioned units, specify ameasuring point, and measure dimensions at the specified point.

Next, description will be made on a method of controlling the electronmicroscope based on a luminance histogram for a sample image. FIG. 3(A)is a diagram showing an example of a luminance histogram for a sampleimage, where the horizontal axis represents the level of halftone, andthe vertical axis represents the number of pixels. The luminancehistogram is differentiated once to create a first-order differentialgraph 302 shown in FIG. 3(B), and twice to create a second-orderdifferential graph 303 shown in FIG. 3(C). Halftone levels a, b, atwhich the second-order differential graph 303 takes maxima, are thoughtto represent the neighborhood of a border between the sample image andbackground in the histogram 301 (hereinafter called the “border point”).In this embodiment, since the there are 256 levels of halftone rangingfrom 0 to 255, the histogram control is required to limit the borderpoints at both ends within the 255 levels of halftone.

FIGS. 4A–4C are diagrams showing an exemplary luminance histogram of asample image created by the sample display unit. As shown in FIG. 4A,the two border points a, b exist on curved sections at both ends of theluminance histogram. The border point a represents a border between thedarkest region of the background and the sample image, while the borderpoint b represents a border between the brightest region of thebackground and the sample image (edges of the sample). The border pointsa, b shown in FIG. 4A are the same as the halftone points a, b at whichthe second-order differential graph 303 of the luminance histogram,described in connection with FIG. 3, reaches the maxima.

Since the histogram control is only required to consider the existenceof the border points a, b at both ends, the histogram 401 issecond-order differentiated in both end regions to find the borderpoints a, b. The image is regarded as highly visible if the two borderpoints exist on the histogram, and the width of the histogram fallswithin the 256 levels of halftone. Therefore, the brightness of thesample image is adjusted by the average value of the histogram, and thecontrast is mainly adjusted by the dynamic range of the amplifier 118 inthe secondary electron detector 117 such that the two border pointsexisting at both extreme ends fall under the 256 levels of halftone.

When the border points exist at both ends of the histogram as in FIG.4A, the brightness B of the image is calculated by:B=S/N  (1)where S is an area when the histogram is integrated, and N is the totalnumber of pixels.

The probe current amount is controlled such that the brightness B isshifted to the position of a halftone level X substantially at thecenter, for example, a halftone level 130. Since the brightness isgenerally proportional to the probe current, the following Equation (2)is established, and the probe current amount I_(P) is found by Equation(3):S/N:I _(O) =X:I _(P)  (2)I _(P) =X·I _(O) ·N/S  (3)where I_(O) is the probe current before the control is conducted, andI_(P) is the probe current at which the brightness B reaches a halftonelevel X.

Next, a minimum frame count, by which the scanning can be made, iscalculated based on the calculated probe current amount. Since there isa relationship between the probe current I_(P) and the required framecount F, represented by I_(P)×F=A (constant), the frame count iscalculated in accordance with this equation. The constant A differs fromone sample to another. The contrast control is conducted by adjustingthe dynamic range of the amplifier 118 such that the halftone width(b−a) between the halftone levels a, b at the border points, found bythe second-order differential, falls within the 256 levels of halftone.

When the sample image is too dark, the resulting histogram shifts to theleft (toward the lower level of halftone) as shown in FIG. 4B, where thenumber of pixels is increased in a lower halftone end region includingthe level 0 of halftone, and a peak appears at the lower end ofhalftone. Conversely, when the sample image is too bright, the histogramshifts to the right (toward the higher level of halftone) as shown inFIG. 4C, where the number of pixels is increased in a higher halftoneend region including the level 255 of halftone, and a peak appears atthe higher end of halftone. In FIG. 4B, the border point a on the lowerhalftone side does not exist on the luminance histogram, while in FIG.4C, the border point b on the higher halftone side does not exist on theluminance histogram. In the following, description will be made on ahistogram control method when the brightness of a sample image is biasedso that one border point does not appear on the histogram, as describedabove.

A histogram 402 shown in FIG. 4B illustrates the case where the sampleimage is short of brightness so that the border point a does not appearon the histogram 402. In this event, a histogram function is calculated,a tangential line 403 is drawn to the curve of the histogram in thelower halftone region, and an intersection a′ of the tangential line 403is located on the halftone level axis. Since the border point a existswithin a range from the halftone level a′ to halftone level 1, a′ isregarded as a virtual border point, and a halftone level at that pointis designated a′ (<0). The tangential line 403 is connected to thehistogram 402 on the lower luminance side to create a virtual histogram.Then, the probe current amount is controlled to a calculated value atwhich the brightness B (=S/N) is equal to a halftone level X. When theprobe currents before and after the control are designated I_(O), I_(P),respectively, the aforementioned Equation (2) is satisfied, so that theprove current amount after the control is expressed by theaforementioned Equation (3).

The minimum frame count F is calculated based on the relationalexpression I_(P)×F=A (constant), in a manner similar to FIG. 4A. Then,the dynamic range of the amplifier 118 is adjusted such that a halftonewidth (b-a′) of the histogram is included in 256 levels of halftone tocontrol the contrast. With this control, the histogram 402 is shifted asindicated by a broken-line histogram 404 in FIG. 4B. Since the histogram404 after the shift includes the two border points, the resulting sampleimage exhibits a high visibility.

A histogram 405 shown in FIG. 4C illustrates the case where the sampleimage is so bright that the border point b does not appear on thehistogram 405. In this event, a tangential line 406 is drawn to thecurve of the histogram 405 in a higher halftone region, and anintersection b′ of the tangential line 406 is located on the halftonelevel axis. Since the border point b exists in a range from halftonelevel 254 to the halftone level b′, b′ is assumed to be a virtual borderpoint. Subsequently, a similar method to that used in FIG. 4B is used tocalculate the probe current amount at which the brightness B (=S/N) of avirtual histogram, which includes the tangential line 406 connected onthe higher luminance side, is equal to a halftone level X by theaforementioned Equation (3) to control the probe current amount. Then,the dynamic range of the amplifier 118 is controlled such that thehalftone width (b′−a) of the histogram is included in the 256 levels ofhalftone to control the contrast. With this control, the histogram 405is shifted as indicated by a broken-line histogram 407 in FIG. 4C. Sincethe histogram 407 after the shift includes the two border points, theresulting sample image exhibits a high visibility.

When a resulting histogram is biased to the lower luminance side orhigher luminance side as shown in FIG. 4B or 4C, a histogram is createdagain from a captured image after the probe current amount is controlledand the dynamic range is adjusted, and it is confirmed whether or notthe resulting histogram is in a desired shape. The confirmation is madefrom the presence or absence of the border points on the createdhistogram, and the brightness. If the histogram is not in the desiredshape, another histogram is created again.

While the foregoing description has been made in connection with aluminance histogram which has one peak, the histogram control can beconducted in a similar manner when a luminance histogram has two or morepeaks. Referring now to FIGS. 5A–5C, description will be made on thehistogram control for a luminance histogram which has two peaks.

FIG. 5A is a diagram showing an exemplary luminance diagram 501 whichhas two peaks p1, p2. Such a luminance histogram is created, forexample, when a high scaling factor is chosen for a sample image tocause an increase in the number of pixels in a sample region (highluminance region), and is regarded as superimposition of two luminancehistograms corresponding to the peaks p1, p2 in the background regionand sample region. Then, border points a, b can be assumed for aluminance histogram corresponding to the peak p1, while border points c,d can be assumed for a luminance histogram corresponding to the peak p2.The border points a, b are located at halftone levels at which asecond-order differential graph of the luminance histogram correspondingto the peak p1 indicates maxima when the luminance histogram 501 isbroken down into two luminance histograms, while the border points c, dare located at halftone levels at which a second-order differentialgraph of the luminance histogram corresponding to the peak p2 indicatesmaxima. Since a halftone range between the two border points appearingat both ends of a luminance histogram is important for making anobservation, a range of a halftone width ab is important for thebackground region of the peak p1, and a range of a halftone width cd isimportant for the sample region of the peak p2. From the foregoing, itcan be said that for the luminance histogram having the two peaks p1,p2, a halftone width required for an observation is a range defined bythe border points a, d at both ends.

In conclusion, even in a luminance histogram having two peaks, thehistogram control only needs to consider the existence of the borderpoints a, d in both end regions of the histogram, so that the histogram501 is second-order differentiated in both end regions thereof to locatethe border points a, d. Then, the histogram control is conducted in thesame manner as described with reference to FIGS. 4A–4C, such that thetwo border points appear on the histogram, and the width of thehistogram falls within the 256 levels of halftone.

When border points appear at both ends of a histogram as shown in FIG.5A, the brightness (average value of the histogram) B of the image iscalculated by the aforementioned Equation (1), when S is an area whenthe histogram is integrated, and N is the total number of pixels. Theprobe current amount is controlled such that the brightness B is shiftedto the position of a halftone level X substantially at the center, forexample, halftone level 130. Since the brightness is generallyproportional to the probe current, the aforementioned Equation (2) isestablished, and the probe current amount I_(P) is found by theaforementioned Equation, where I_(O) is the probe current before thecontrol is conducted, and I_(P) is the probe current at which thebrightness B reaches a halftone level X. The frame count F is alsocalculated based on the probe current amount using the relationalequation I_(P)×F=A (constant) in a manner similar to the foregoing. Theconstant A differs from one sample to another. The contrast control isconducted by adjusting the dynamic range of the amplifier 118 such thatthe halftone width (d−a) between the halftone levels a, d at the borderpoints, found by the second-order differential, falls within the 256levels of halftone.

When the sample image is too dark, the resulting histogram shifts to theleft (toward the lower level of halftone) as shown in FIG. 5B, where thenumber of pixels is increased in a lower halftone end region includinghalftone level 0, and a peak appears at the lower end of halftone.Conversely, when the sample image is too bright, the histogram shifts tothe right (toward the higher level of halftone) as shown in FIG. 5C,where the number of pixels is increased in a higher halftone end regionincluding halftone level 255, and a peak appears at the higher end ofhalftone. In FIG. 5B, the border point a on the lower halftone side doesnot exist on the luminance histogram, while in FIG. 5C, the border pointd on the higher halftone side does not exist on the luminance histogram.A similar histogram control method to that employed when a luminancehistogram has one peak is also employed when the brightness of a sampleimage is biased so that one border point does not appear on thehistogram, as described above.

Specifically, for a histogram 502 shown in FIG. 5B, a histogram functionis calculated, a tangential line 503 is drawn to the curve of thehistogram in the lower halftone region, and an intersection a′ of thetangential line 503 is located on the halftone level axis. Theintersection a′ is regarded as a virtual border point, and a halftonelevel at that point is designated a′ (<0). The tangential line 503 isconnected to the histogram 502 on the lower luminance side to create avirtual histogram. Then, the probe current amount is controlled to acalculated value at which the brightness B (=S/N) is equal to a halftonelevel X. When the probe currents before and after the control aredesignated I_(O), I_(P), respectively, the aforementioned Equation (2)is satisfied, so that the prove current value after the control isexpressed by the aforementioned Equation (3).

The minimum frame count F is calculated based on the relationalexpression I_(P)×F=A (constant). Then, the dynamic range of theamplifier 118 is adjusted such that a halftone width (d−a′) of thehistogram is included in the 256 levels of halftone to control thecontrast. With this control, the histogram 502 is shifted as indicatedby a broken-line histogram 504 in FIG. 5B. Since the histogram 504 afterthe shift includes two border points, the resulting sample imageexhibits a high visibility.

A histogram 505 shown in FIG. 5C illustrates the case where the sampleimage is so bright that the border point d does not appear on thehistogram 505. In this event, a tangential line 506 is drawn to thecurve of the histogram 505 in a higher halftone region, and anintersection d′ of the tangential line 406 is located on the halftonelevel axis. The intersection b′ is assumed to be a virtual border point.Subsequently, a similar method to that used in FIG. 5B is used tocalculate the probe current amount at which the brightness B (=S/N) of avirtual histogram, which includes the tangential line 506 connected onthe higher luminance side, is equal to a halftone level X by theaforementioned Equation (3) to control the probe current amount. Then,the dynamic range of the amplifier 118 is controlled such that thehalftone width (d′−a) of the histogram is included in the 256 levels ofhalftone to control the contrast. With this control, the histogram 505is shifted as indicated by a broken-line histogram 507 in FIG. 5C. Sincethe histogram 507 after the shift includes two border points, theresulting sample image exhibits a high visibility.

When a resulting histogram is biased to the lower luminance side orhigher luminance side as shown in FIG. 5B or 5C, a histogram is createdagain from a captured image after the probe current amount is controlledand the dynamic range is adjusted, and it is confirmed whether or notthe resulting histogram is in a desired shape. The confirmation is madefrom the presence or absence of the border points on the createdhistogram, and the brightness. If the histogram is not in the desiredshape, another histogram is created again.

FIG. 6 is a flow chart illustrating a procedure of image acquisitionbased on the histogram control described above, and a measurement(length measurement) using this image. First, a sample image is observed(S11). Next, a luminance histogram is created from the sample image(S12). A second-order differential graph is created from the luminancehistogram (S13), and it is determined whether or not border points ofthe background and sample appear on the lower luminance side and higherluminance side, respectively (S14). If any of the border points does notappear, a virtual border point is calculated as described in connectionwith FIG. 4B, 4C or FIG. 5B, 5C (S15). Subsequently, the probe currentamount is calculated such that the brightness of the image issubstantially equal to a median value of the halftone range, and theframe count is calculated corresponding to the probe current amount(S16). The calculated probe current amount and frame count may bedisplayed on a monitor screen. Next, the dynamic range of the amplifier118 is adjusted to achieve an appropriate contrast (S17). Finally, asample image is formed using the probe current amount and the framecount calculated at step 16 to make a length-measurement (S18).

FIG. 7 is a diagram illustrating an exemplary display screen of thesample image display unit 121. In this example, the user can confirm theprobe current amount 701, the frame count 702, and a re-createdhistogram 703 on the display screen. Border points and brightness areassociated with the displayed histogram 703, allowing the user toconfirm the histogram. A sample image for length measurement isdisplayed in a window 704.

The histogram adjusting method described above can provide a histogramin which border points of a background and a sample appear within ahalftone range of levels 0–255. Then, the prove current amount whichprovides an appropriate brightness for the sample image can becalculated based on the border points, and a minimum frame count can becalculated in accordance with the probe current amount, thus controllinga highly visible sample image.

When length measurements were made ten times in accordance with the lowframe scanning scheme of the present invention with the number of framesreduced from conventional 16 to four and the probe current amountincreased from 8 pA to 25 pA, the amount of slimming could be reducedfrom 6.7 nm to 3.0 nm. When the second length measurement was made, theslimming could be reduced from 1.3 nm to 0.3 nm. The length measurementrepeatable accuracy (3σ) is 0.7 nm which satisfies an allowable range.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A scanning electron microscope comprising: an electron beam source; alens system for converging a primary electron beam emitted from saidelectron beam source onto the surface of a sample; a scanning unit fortwo-dimensionally scanning the converged primary electron beam on thesurface of the sample; a detector for detecting a secondary signalgenerated from the sample irradiated with the primary electron beam; anamplifier for amplifying a signal detected by said detector; a renderingunit for forming a sample image based on the signal amplified by saidamplifier; and a control processing unit, said control processing unitconfigured to create a luminance histogram of a sample image, control acurrent value of the primary electron beam such that the brightness ofthe image calculated from the luminance histogram is locatedsubstantially at the center of an overall halftone width, and control anamplifying condition for said amplifier to provide a contrastcorresponding to the brightness of the image.
 2. A scanning electronmicroscope according to claim 1, wherein said control processing unitderives a function from the luminance histogram, draws a curverepresentative of the function, calculates two halftone levels at whicha second-order differential of the function takes maxima in a lowerhalftone region and a higher halftone region of the curve, and controlsthe amplifying condition for said amplifier such that the two halftonelevels are included in a predetermined halftone range.
 3. A scanningelectron microscope according to claim 1, wherein: when the curverepresentative of the function derived from the histogram has a peak ina lower halftone end region, said control processing unit creates avirtual histogram by replacing the peak region of the histogram with atangential line which is tangential to a lower halftone region of thecurve and intersects with a halftone level axis, and controls thecurrent value of the primary electron beam such that the brightness ofthe image calculated from the virtual histogram is located substantiallyat the center of the overall halftone width.
 4. A scanning electronmicroscope according to claim 3, wherein said control processing unitcontrols the amplifying condition for said amplifier such that ahalftone level at which the tangential line intersects with the halftonelevel axis, and a halftone level at which the second-order differentialtakes a maximum in a higher halftone region of the curve are included inthe predetermined halftone range.
 5. A scanning electron microscopeaccording to claim 1, wherein: when the curve representative of thefunction derived from the histogram has a peak in a higher halftone endregion, said control processing unit creates a virtual histogram byreplacing the peak region of the histogram with a tangential line whichis tangential to a higher halftone region of the curve and intersectswith a halftone level axis, and controls the current value of theprimary electron beam such that the brightness of the image calculatedfrom the virtual histogram is located substantially at the center of theoverall halftone width.
 6. A scanning electron microscope according toclaim 5, wherein said control processing unit controls the amplifyingcondition for said amplifier such that a halftone level at which thetangential line intersects with the halftone level axis, and a halftonelevel at which the second-order differential takes a maximum in a lowerhalftone region of the curve are included in the predetermined halftonerange.
 7. A scanning electron microscope according to claim 1, whereinsaid control processing unit calculates a frame count based on thecurrent value of the primary electron beam.
 8. A scanning electronmicroscope according to claim 7, further comprising a display unit fordisplaying the luminance histogram of the sample image, the currentvalue of the primary electron beam, and the frame count.